This invention relates to a pressure gas engine having improved speed control means for throttling very precisely a gas valve in which the control means comprises a bellows anchored at one end with the other end attached to a movable valve and with both sides of the bellows exposed to the internal gas pressure together with a speed responsive device for controlling flow of bleed gas from one side of the bellows to control precisely the speed at any particular speed setting of a variable trigger. A centrifugally responsive means is provided comprising a spring member having a weighted centrifugally responsive portion arranged at an angle to a vent valve member movable for controlling bleed gas flow from the one side of the bellows and a mounting member on which the spring member is mounted and including support means engaged by the weighted portion at speeds high enough to damage an unsupported spring member.